This invention relates generally to vortex-type flowmeters adapted to operate in conjunction with a common external sensor coupled to a digital indicator whereby the same sensor and indicator system may be used to take readings from a large number of installed flowmeters, and more particularly to an arrangement which automatically compensates for differences in individual meter factors so that accurate readings are obtainable from all of the flowmeters despite these differences.
Artificial lift expedients are often required to increase oil recovery at oil well sites. One widely used form of secondary recovery is the water-flood technique wherein pressurized water is forced through an injection well adjacent the site of the producing well, the injected water flooding the oil bearing region and providing the necessary pressure for oil extraction. In a secondary recovery system, oil intermingled with water is yielded by the producing well. The water is thereafter separated from the oil and is returned to injection pumps delivering water to several injection wells, so that the secondary recovery system involves a network of water lines leading to a group of functioning wells. Waterflood techniques are also currently in use in uranium mining.
In maintaining and servicing a waterflood system, it is necessary to periodically check the water flow rate at various points in the water line network. The present practice is to effect measurement by means of turbine meters installed in the water lines. In the conventional turbine meter, the turbine rotor is mounted within the flow conduit, a permanent magnet being incorporated in the rotor. The rotating magnet induces an alternating-current in a pick-up coil located in the external housing of the meter, the frequency of the generated signal being proportional to the volumetric flow rate. The frequency of the signal is converted into a reading of flow rate.
Since turbine meters are relatively expensive, and a waterflood system requires a large number of such meters, one recent innovation has been to omit the pick-up coil from the meter and to provide a separate pick-up coil coupled to a battery-operated test set which affords a flow rate reading. This practice is feasible since it is only occasionally necessary for an operator to check flow rate at the meter installation and then, if necessary, to make a manual valve adjustment to correct flow rate. Thus the operator who carries the pick-up coil and the test set makes a tour of the various turbine meter installations to check the flow rate.
The main drawback of turbine flowmeters in the context of a waterflood system is that because it has a rotor which is exposed to the water, there is a reliability problem in that the water being measured is often dirty and tends to foul and degrade the rotor and its bearings, particularly if the water contains abrasive particles and corrosive chemical constituents. Hence after prolonged use, the turbine meter may become inoperative or inaccurate.
In my copending application, above-identified, there is disclosed a vortex-type flowmeter adapted to operate in conjunction with an external sensor coupled to a portable digital read-out device whereby the same external system may be used to take readings from a large number of installed flowmeters. The installed flowmeters are therefore altogether devoid of electrically-powered devices so that no danger exists in environments that cannot tolerate unattended electrical circuits.
The flow meter disclosed in the copending application includes a flow tube forming a passage for the fluid to be metered and an obstacle assembly disposed in the tube and capable of generating strong fluidic oscillations which cause a deflectable section of the assembly to vibrate at a corresponding rate. Disposed within the deflectable section is a rod which is caused to vibrate at the same rate, the rod vibration being transferred to a probe placed within a nondeflectable section of the obstacle assembly and extending to the exterior of the tube, whereby the vibrations of the deflectable section within the conduit are transmitted to the exterior thereof.
The probe extension terminates in a coupling head which is engageable by a sensor adapted to convert the probe vibrations into a corresponding electrical signal whose frequency is proportional to flow rate. The sensor is coupled to a test set serving to convert the signal into a flow rate reading. Such meters will hereinafter be referred to as external-sensor vortex-type flowmeters.
As a practical matter, it is virtually impossible to manufacture on a large scale external-sensor vortex-type flowmeters possessing identical meter factors. The meter factor represents the number of cycles generated per gallon of fluid passing through the flow tube. The calibration curve for a vortex-type flowmeter is produced by plotting the Reynolds number of the flow tube against the meter factor.
In making flowmeters on a mass-production basis, variations in the flowmeter structure invariably give rise to different meter factors. For example, manufacturing tolerances in the commercial piping or forgings used for making the meter are such that no two flow tubes have exactly the same diameter or other parameters. Hence when the same sensor and digital indicator system is used in conjunction with a group of flowmeters, the respective readings will be erroneous to an extent determined by the degree to which the actual meter factor of the meter being read deviates from the nominal meter factor for which the system is designed.
Another problem encountered in vortex-type flowmeters used in water flood applications is in regard to meter accuracy or linearity. Because the flow rates in this application are relatively low, the meters designed for measuring these rates are quite small and somewhat non-linear. Hence even if the flowmeter reading is corrected for meter factor, the reading is still lacking in accuracy.